Traditionally, the Voltage Standing Wave Ratio (VSWR) of antenna systems is measured using filtered or narrow-band power measurements. By measuring the power of the forward and reflected signals (Pfwd and Prefl) simultaneously, the scalar return loss (RL) and reflection co-efficient (Γ) can be calculated.
Typical VSWR measurement systems consist of a directional coupler, and analog power detectors to measure the voltage of the forward signal, denoted Vfwd, and the voltage of the reflected signal, denoted Vrefl. A mixer circuit and low pass filter are used to measure the VSWR over a specific frequency and bandwidth. An analog to digital converter (ADC) quantizes the power detector value. High values of Vrefl relative to Vfwd indicate potential transmission line and antenna faults. Both Vrefl and Vfwd will be affected if there is an interference signal coupled into the measurement within or even adjacent to the measurement bandwidth.
The current analog VSWR (AVSWR) and narrowband digital VSWR (DVSWR) measurement systems are unable to detect or account for interferers as they have no knowledge of the dynamic interferer. False alarms can be raised if the interference is large enough to raise the Vrefl such that it looks like a cable fault. False alarms may cause outages in the network if the transmission signal is turned off by fault management software to prevent damage to the radio in the event of a cable fault. False alarms also may cause unnecessary maintenance on the radio and antenna system. Therefore, special tests are used to verify the sensitivity of the AVSWR measurement to adjacent interferers.
Another drawback is that the AVSWR measurement can only be performed over a small bandwidth determined by the configured frequency of the mixer and low pass filter. The AVSWR method cannot measure the Vfwd or Vrefl separately. Any voltage induced on the antenna system due to an interferer will be coupled into both the Vfwd and Vrefl signals, and will impact the accuracy of the measurement. The level of coupling is based on the S-parameters of the directional coupler.
The AVSWR measurement method also does not compare the Vfwd or Vrefl with the original transmitted signal, denoted Vref, and thus does not determine if there is any interference that impacts the overall accuracy of the measurement.